Magnetic needle separation and optical monitoring

ABSTRACT

Apparatuses and methods for removing magnetic particles from suspensions are described. One embodiment of the apparatus is called a magnetic needle.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 15/937,705,filed 27 Mar. 2018, which was a continuation of application Ser. No.14/679,216, filed Apr. 6, 2015, now U.S. Pat. No. 9,964,469, issued 8May 2018, each of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention (Technical Field)

The present invention relates generally to methods and apparatus forpurifying or concentrating samples using magnetism.

Magnetic particle suspensions have been proposed for use in chemical andbiological assays as well as biomedical research and clinicalapplications. For these applications, it can be important that thesuspensions be free of excess reagents used in their synthesis orreactions that introduce surface functionality using polymers,biological molecules, etc. Residual impurities in the suspensionfollowing a reaction can alter the reactivity of the nanoparticles indownstream applications and can induce toxic side effects in biologicalsamples. Typical particle separation methods includeultracentrifugation, column chromatography, dialysis, etc. These methodsrequire either the use of expensive equipment or a large quantity ofconsumable materials and reagents.

SUMMARY OF THE INVENTION

The use of a magnetic needle as described herein to collect magneticparticles from a non-magnetic solution can provide a straightforward,cost-effective approach to separation. The properties of the magneticneedle scale with size, so that the construction of the needle can bevaried to accommodate reactions of different volumes with maximumefficiency. The collected nanoparticles can be removed from thedispersant and resuspended in the medium of choice at the desiredconcentration. By monitoring the separation optically, the duration ofthe separation, the separation efficiency, and the quality of thesuspension can also be assessed.

An example embodiment of the present invention provides an apparatuscomprising a rod having a distal end and a proximal end with one or morerare-earth magnets located at the distal end of the rod, the rod capableof being inserted into and retracted an environment containing magneticparticles. The apparatus further includes a sheath that is removablyattached to the rod and covers at least a portion of the rod thatextends into the environment.

While the apparatus is in the environment, the magnetic particles aremagnetically collected against the sheath of the rod over a period oftime. The apparatus is separated from the environment with the particlesmagnetically attached to the sheath. The sheath having the magneticparticles attached thereto is removed from the rod. Removal of thesheath from the rod removes the magnetic attraction that attracted theparticles to the apparatus, and the particles can then be separated formthe sheath if desired, for example by washing the sheath.

In an example application, the magnetic collection time ranges fromabout 30 seconds to about 240 seconds.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a graphical illustration of a magnetic nanoparticle whichcontains a ferrite core, a coating of a biocompatible material, and acoating of specific antibodies according to one embodiment of thepresent invention.

FIG. 2 illustrates three views (FIG. 2a , FIG. 2b , and FIG. 2C) ofthree magnetic separation devices according to different embodiments ofthe present invention.

FIG. 2a illustrates a magnetic biopsy device comprising a guidance tubewith a central needle containing a stainless steel rod and a magnetictip on the end of the rod wherein the magnetic tip ranges in size fromone (1) cm to five (5) cm in length according to one embodiment of thepresent invention.

FIG. 2b is a schematic illustration of an example magnetic biopsy devicecomprising a guidance tube with a ferrous needle attached at one end toa strongly magnetized material according to one embodiment of thepresent invention.

FIG. 2c is a schematic illustration of an example magnetic biopsy devicecomprising a guidance tube with a ferrous material attached at one endin addition to a strong electromagnet attached thereto according to oneembodiment of the present invention.

FIG. 3 is a schematic illustration of a calculation showing the magneticfields in the vicinity of the magnetic needle.

FIG. 4 is a schematic illustration of a calculation showing the forcefields on magnetic nanoparticles in the vicinity of the magnetic needle.

FIG. 5 is a schematic illustration of the results of a calculationshowing the time it takes to pull magnetic nanoparticles onto a magneticbiopsy device as a function of the magnetic nanoparticles initialdistance from the needle when the biopsy device is inserted into theenvironment.

FIG. 6 is a schematic illustration of an example embodiment of thepresent invention comprising a magnetic needle insert with a sheath.

FIG. 7 is a schematic illustration of magnetic field contour lines for arare-earth magnet.

FIG. 8 is a schematic illustration of calculated collection times forvarious volumes of cells in suspension.

FIG. 9 is a schematic illustration of an example apparatus according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION

As used herein “a” and “an” means one or more.

According to an example embodiment of the present invention, a magneticapparatus assembly is provided, similar to a conventional needle with acentral shaft and a thin-walled cylindrical channel surrounding theshaft.

Magnetic fields of an example embodiment of the present inventionproduce force fields to attract magnetic particles.

Superparamagnetic nanoparticles have the property that, if no magneticfield is present, (a field smaller or comparable to the earth's field),the particles behave paramagnetically, i.e. they have a permeabilityclose to one. However, in a relatively strong magnetic field, theybecome aligned with that field and exhibit ferromagnetism withpermeabilities of several thousand, depending on the particle propertiesand the strength of the applied magnetic field. This very unusualproperty means that they can be injected into a volume when there is noapplied field without congealing through their own magnetic attraction,but will be strongly attracted to regions of a large applied field.Other bio-materials in the area that are paramagnetic are not affected.

Referring now to FIG. 1, the concept of a magnetic nanoparticle coatedwith an antibody is illustrated according to one embodiment of thepresent invention. Superparamagnetism considerably enhances thecollection, using embodiments of the present invention, of cells thathave been coated with particles such as those in FIG. 1. According toone embodiment of the present invention, a antibody-labelednanoparticles are introduced to a sample containing cells to which theantibodies can bind. After waiting for a specified period, amagnetically tipped rod producing a strong magnetic field is insertedinto the sample and left for a pre-determined collection time. The rod(also known as a wire) is removed with the cells containing the magneticnanoparticles attached thereto. The nanoparticles are collected from thetip of the wire by either a strong magnetic field on the tip, or othermechanisms as described below.

Referring now to FIG. 2a , a tip of magnetic material of about 1 cm islocated at one end of the central removable rod is illustrated accordingto another embodiment of the present invention. According to thisconfiguration, magnetic material is located at the end of the rod atabout the last cm of the central rod length. Magnetic material such asiron, or rare earth materials such as Knife, Smock, Ceramic, and Alnicocan be used in these magnetic rods. Magnetic field intensity maximumscan be about 4000 G for Ceramic 5 and about 13,000 G for Knife-42H withparticular values of about 4000 G at the surface of the Knife magnets.The magnetic needle tip is of about one (1) mm diameter and about ten(10) mm length. The magnetic material can be located at any positionalong the length of the rod. The dimension of the magnetic needle canrange in diameter from about 0.5 mm to about 10 mm. The dimension of themagnetic needle can range in length from about 1 mm to about 1000 mm.

Referring now to FIG. 2b , a magnetic needle with a removable magnet isillustrated according to an example embodiment of the present invention.This permits easy removal of the attached nanoparticles from the needleafter extraction. In this embodiment, the needle is conically enlargedas it extends beyond the housing of a tube wall or cannula and proceedsoutside and becomes physically in contact with a larger magnet. Thisstructure permits a concentration of magnetic lines from the magnetlocated at one end of the rod along the length of the rod to the smalltip at the opposing end.

The material and geometry concentrate significant flux to the tip. Arare earth magnet is attached to the large end so that its magnetic fluxlines will proceed through the needle and are emitted at the tip end. Inthis example embodiment of the present invention, large rare earthmagnets can be used at the end of the rod that is opposite the insertionpoint, which is not inserted into the sample. The external magnet can beremoved when the needle is extracted. In the absence of the magneticfield, the nanoparticles are removed from the needle by an additionalexternal magnet extraction.

Referring now to FIG. 2c , an electromagnetic needle is illustratedaccording to an example embodiment of the present invention. In thisexample embodiment an electromagnetic coil wound around a ferromagneticcore and located at the end of the inserted magnetic needle produces themagnetic field. The external magnet field can be turned off when theneedle is extracted to remove the nanoparticles from the needle by anadditional external magnet extraction.

According to an example embodiment of the present invention, thecompatibility of the needle (also known as a wire or rod) that ismagnetizable is considered to avoid contact of rare earth magneticmaterial with the sample. The needle material can be coated with a thinplastic coating that is compatible with the sample. This coating alsopermits sterilization procedures and potential reuse of the needle.

In another embodiment of the present invention, an external magnet canbe used to magnetize the magnetizable rod. The external magnet is usedto increase the magnetic force on the particles in the vicinity of theneedle. The external magnetic pole can be either permanently magnetizedor electromagnetically excited. This magnetic circuit decreases theparticle collection time at the needle. A rare earth or electromagneticmagnet is placed on the opposite side of the collection location toincrease the magnetic divergence vector field at the needle locationresulting in increase efficiency in collecting magnetic particles.According to an example embodiment of the present invention, theexternal magnet is similar to a commonly used C-magnet where in thiscase the C surrounds the collection volume and the needle is attached toone of the C-magnet faces. A Knife magnet, or alternatively anelectromagnetic coil giving field strength of about 0.6 T, can be usedto generate the magnetic field.

FIG. 3 illustrates a magnetic field and the field derivatives of themagnetized needle extending to reasonable distances with the magnetizingfields from these magnets according to an example embodiment of thepresent invention. Magnetic field strength from the needle versusdistance from needle in a two (2) cm by two (2) cm box.

Referring now to FIG. 4, the forces exerted on magnetic nanoparticlescan be calculated using electromagnetic theory. Assuming that theviscosity of the medium is close to water and using Stokes theorem, itis possible to calculate the effective range of attraction and the timeto pull nanoparticles to the needle. Magnetic lines of force on magneticnanoparticles from the magnetic needle in a two (2) cm by two (2) cmbox.

Referring now to FIG. 5, detailed calculations of the forces, ranges,and collection times for various field strengths, needle configurations,and magnetic nanoparticles properties are illustrated in the graph. Thetime required to pull cells labeled with nanoparticles to the needle toprovide the necessary guidance for collection times of thesuperparamagnetic particles are calculated according to an exampleembodiment of the present invention.

Large number of magnetic nanoparticles attach to each cell through theantibody mechanism and this adds considerably to the ability to attractthe cells to the needle. For example, values of 2×10⁷ nanoparticles percell or 30 PG of superparamagnetic iron per cell are achievable. Therange of the magnetic needle for attracting magnetic nanoparticles iscalculated by modeling the magnetic needle used to collect the cellscontaining the nanoparticles as a magnetic dipole of dipole moment{right arrow over (m)}.

In an example application, human cancer or other cells are obtained frombone marrow biopsies from clinical examinations of patients suspected ofbone marrow disease or metastasis. Magnetically-labeled antibodiesdirected against CD34 (leukemia cells and myelodysplasia marker) andcytokeratin (breast cancer marker) are one example of nanoparticlesutilized. The level of normal cells expressing cytokeratin in normalbone marrow is negligible, and the level of normal cells expressing CD34is significantly below 1% in normal marrows. Since tumor involves levelsupwards of 100% of the marrow, these markers will detect and sampletumor cells preferentially, if the tumor is present. This significantlyincreases the specificity of the bone marrow biopsy.

Referring now to FIG. 6, one embodiment of the present inventioncomprises magnetic biopsy apparatus 100 for collecting a biopsy sampleof diseased cells from a subject with sheath 10. This embodimentpreferably comprises cannulae 20 for penetrating a body of a subjecthaving openings at the distal end and proximal end with thecross-sectional area in between the distal end and the proximal endsubstantially hollow for injecting superparamagnetic nanoparticles intoa biopsy site of a potential through cannulae 20 which contacts thepatient at the distal end of the apparatus 100. The superparamagneticnanoparticles are labeled with a receptor for a specific target ofinterest of a diseased cell. The superparamagnetic nanoparticles do notattract each other in the absence of a magnetic field.

This embodiment further comprises rod 14 having a distal end and havingat least one rare earth magnet 12 located at the distal end of rod 14,rod 14 capable of being inserted into and retracted from cannulae 20with rare earth magnet 12 positionable partially outside of the proximalend of cannulae 20. Sheath 10 is preferably removable attached to rod 14at the distal end and covers the portion of rod 14 where rare earthmagnet 12 is located. Sheath 10 is preferably polyimide. In oneembodiment the sheath 10 is removable attached to the rod 14. The sheath10 is attached to the rod 14 when the rod 14 is inserted into the biopsysite. The superparamagnetic nano particles are attracted to the rareearth magnets 12 on the rod 14 and are localized on the sheath 10 whichis positioned between the rare earth magnet 14 and the superparamagneticnano particles.

According to one example the rod assembly provides that the length ofthe rod may be about 18.5 mm and be made of any biocompatible materialfor example grade 316 stainless steel. The rod can be held in a housingwhich can run the length of the rod or some portion thereof for examplethe housing can have a length of about 14 mm and cover the distal tip ofthe needle where the one or more rare-earth magnets are located. Therare-earth magnets can be of any type for example NdFeB. The housing ora portion thereof can be made from a biocompatible housing for examplepolyimide. The diameter of the rod assembly can be of any diameter.According to one embodiment the diameter of the rod assembly is about0.108 mm.

A sheath can cover the housing at the distal end of the assembly wherethe rare-earth magnets of the rod are located. The sheath can cover onlya portion of the rod and have for example a length of about 4.5 mm. Thesheath can be removable attached to the housing or to the rod when thehousing is not present.

The one or more rare-earth magnets located at the tip of the needle canbe about 2 mm in length and 1 mm in diameter and separated from eachother by about 2 mm according to one embodiment of the presentinvention. However, the size of the magnets is not limiting and othersize magnets can create a similar magnetic force. For example themagnets can range in size from 0.5 to 4 mm. These dimensions andmaterials are for illustration purposes only and the embodiments of thepresent invention are not limited to these dimensions or materials.

Example

Magnetic nanoparticles conjugated with antibodies for targeting specificeptitopes on an ALL surface, such as a cell surface receptor like CD34are incubated with a population of cells to be assayed.

A magnetic needle surrounded by a sheath such as plastic according toone embodiment of the present invention is introduced into the cell andmagnetic nanoparticle mixture. A plurality of rare-earth magnets at thedistal end of the needle attracts magnetic nanoparticle bead complexesas well as nanoparticles uncomplexed to cells. The sheath may extend thelength of the magnetic needle or may cover the needle at only a portionof the length. In one embodiment, magnets are about 2 mm in length andare spaced apart from each other at a distance of about 2 mm.Alternatively, a second sheath may surround this assembly for collectionof nanoparticles and then removed with the nanoparticles subsequentlyplaced in a solution for microscopic or other type measurements.

Magnetic nanoparticles incubated with target cells and forming a complexare attracted to the magnetic needle when the magnetic needle is putinto close proximity to the magnetic nanoparticles. The magneticnanoparticles through magnetic interaction with the rare-earth magnetslocate to the plastic sheath at the distal end of the magnetic needle.Cells bound to the nanoparticle through the antibody are co-located tothe needle with the nanoparticle via magnetic forces. The magneticneedle with the magnetic nanoparticles bound thereto are withdrawn fromthe assay environment. The nanoparticle/cell complexes are removed fromthe sheath for examination and analysis. For example, thenanoparticles/cell complexes removed from the sheath are analyzed with aSQUID sensor platform wherein the decaying remanence fields from thelabeled cells after termination of the magnetic pulse is determined. SeeFlynn et al, in Use of a SQUID Array To Detect T-cells With MagneticNanoparticles In Determining Transplant Rejection, Journal of Magnetismand Magnetic Materials, 311, p 429 (2007).

In the present example CD34 Antibodies (Ab) that are specific to commonALL cells were conjugated to magnetic nanoparticles. One or more (forexample, thousands of nanoparticles) may be attached to each cellthereby producing a large magnetic moment for attraction to the magneticneedle.

Referring now to FIG. 7, a magnetic field contour line graph of themagnetic needle magnet (solid gray bar) and comparison data isillustrated: FIG. 7 contains the magnetic field gradient lines ascalculated from the theory. The cells attracted by the magnet will tendto follow these lines. This has been confirmed by movies of particlemovement.

FIG. 8 illustrates calculated collection times for various volumes usingseveral values of F, where F is for various field strengths of themagnets, from the equations above, representing different viscosities.Experimental data is shown using ficoll and blood to simulate verydifferent viscous media.

Insertion of the magnetic rod in a vial containing recently extractedbone marrow diluted with blood produces a sheath having rings where thecells have been collected due to their complexation with the magneticnanoparticles. These rings correspond to the regions of highestgradients as shown in the figure above.

In one embodiment of the present invention, the magnetic rod is amagnetic needle. It was unexpectedly observed that a ten fold increasein detection of target cells were observed with the magnetic needle overstandard pathology techniques. A standard pathology cell count of 200cells, noting how many of these were blast cells was conducted for bothblood spiked with U937 cells and with fresh bone marrow from leukemiapatients. Each case was diluted with blood by factors of two untilreduced by 1024 in order to simulate a MRD situation where there will bemuch fewer blast cells. This was done with the diluted preparation(after nanoparticles were added and incubated), followed by a one minuteplacement of the magnetic rod in the preparation with a subsequentremoval of the collected cells. Table I and Table II give the resultsfrom blood spiked with U937 cells and for bone marrow samples from humanleukemia cell donors diluted with normal bone marrow.

Table I illustrates the enhancement produced by the rod draws for thenumber of leukemia cells collected on the rod compared to those directlyfrom the cell preparations results of Table II. A 10 fold increase inthe sensitivity for MRD is seen in the rod draws for U937 cells whilethe number of cells in marrow is magnified by a factor of 5.4 overtraditional pathology methods.

TABLE I Dilution U937 Count Needle Count Needle Increase 1/1 200/200 200/200  NA 1/2 127/200  144/200  1.13 1/4 71/200  94/200 1.32 1/844/200  78/200 1.77 1/16 28/200  49/200 1.75 1/32 13/200  32/200 2.461/64 6/200 21/200 3.50 1/128 4/200 12/200 3.00 1/256 2/200  7/200 3.501/512 1/200  8/200 8.00 1/1024 1/200 10/200 10.00

TABLE II Dilution Marrow Count Needle Count Needle Increase 1/1 192/200 194/200 1.01 1/2 169/200  189/200 1.12 1/4 148/200  151/200 1.02 1/8123/200  139/200 1.13 1/16 96/200 119/200 1.24 1/32 79/200 103/200 1.301/64 64/200  99/200 1.55 1/128 54/200  82/200 1.52 1/256 39/200  79/2002.03 1/512 26/200  69/200 2.65 1/1024 11/200  59/200 5.36

Example

In production or collection of magnetic particles, it can be importantto purify or concentrate samples by removing magnetic particles from aliquid suspension. A magnetic apparatus like those described herein canbe used to facilitate rapid separation magnetic particles from organicor aqueous suspensions in either a batch or continuous flow format.Optical monitoring of solution turbidity can be used to monitor theseparation process for optimal duration.

A magnetic apparatus for this example application can comprise permanentmagnets arranged coaxially along a rod, and the configuration secured atthe distal ends with a locking component comprised of a plate, a nut, orsimilar retaining structures. The locking mechanism can be configured topermit the adjustment of the position of the magnets along the axis.Adjusting the position of the magnets, and thus the spacing betweenmagnets, further allows variation of the resulting magnetic fieldgradient. The size of the magnets used and the corresponding spacingbetween magnets can be selected based on (1) the volume being sampledand (2) the dimensions of the sample container, and (3) the propertiesof the particle suspension (including, as examples, one or more ofparticle size, concentration, colloidal stability of the suspension).The apparatus can comprise a magnet encased in a removable, disposablesheath that prevents direct contact between the apparatus and thesample. The properties of the sheath can be chosen for compatibilitywith the solution and the nanoparticles.

In a batch or semi-batch application, a magnetic needle can be comprisedof a solid cylindrical apparatus that is submerged into a sample with astatic volume. For batch operation, a magnetic needle apparatus can besubmerged into solution until separation of particles from the solutionoccurs. Residence time of the magnetic needle in the suspension can bevaried for desired separation of magnetic particles from solution.Following separation of the particles from solution, the sheathed needleand magnetically bound particles can be removed to a secondarycontainer. Alternatively, the magnetic needle apparatus can remain fixedin position and the solution can be drained from the container bygravity or by pumping, leaving magnetic particles bound to the sheathedmagnetic needle apparatus. In either embodiment, removal of the magneticneedle apparatus from the sheath, optionally combined with rinsing usingan appropriate solvent releases the particles from the sheath. Theparticles can be re-suspended at the desired concentration in thesolvent of choice. Additional separations using the needle can beperformed for further purification or concentration.

In a continuous flow or semi-batch application, a magnetic needle cancomprise annular, rather than solid, magnets. A tube can be insertedthrough axially aligned annuli to allow continuous, gravity driven orpumped flow of a suspension containing magnetic particles. The flow ratecan be configured to determine the desired residence time of theparticles in the magnetic needle apparatus. For semi-batch use, a volumeof suspension containing magnetic particles can be introduced intoannular space, with flow out of the space prevented by the use of adevice such as a valve or stopper. Residence time of the suspension inthe annular space can be selected for desired separation of magneticparticles from solution. Following separation of the particles fromsolution, opening of the valve or stopper apparatus allows the solutionto be drained, leaving magnetic particles bound to the tubing within themagnetic needle apparatus. Removal of the tubing from the magneticneedle apparatus to a secondary container, optionally combined withrinsing using an appropriate solvent, releases the particles from thetubing. The particles can be re-suspended at the desired concentrationin the solvent of choice. Additional separations using the needle can beperformed for further purification or concentration.

The progress of the separation can be monitored with a probe thatmeasures the optical density or turbidity of the solution. The probe canbe used with an external device, such as a spectrometer, to monitor thesolution in real time. The rate of change in the turbidity of solutioncan reflect the size of particles in solution, the presence ofaggregates, and completion time of separation.

In an example embodiment, one can measure the time-dependent intensityof transmitted visible light through a colloidal suspension during themagnetic separation process. FIG. 9 is a schematic illustration of anexample embodiment of the invention. A visible light source isexternally coupled to the sample container. Light transmitted throughthe sample solution is measured by an externally coupled detectorpositioned at 180° to the light source. The visible light source can becomprised of a tungsten light source coupled to an optical fiber. Thedetector can be a spectrometer capable of measuring wavelengths between400 and 600 nm. The position of the magnetic needle apparatus should beadjusted so as not to be in the path of the transmitted light. Theintensity of transmitted light as a function of separation time can berecorded and plotted in real time.

The transmission of visible light through a suspension containingmagnetic particles will be low with respect to a solution containing nomagnetic particles. Thus, the intensity of transmitted light willincrease over the course of a magnetic separation.

The intensity (I) as well as the rate of change of intensity (dl/dt) canbe used to accomplish several objectives, such as those described below.

Monitor the progress of the separation: the intensity of transmittedlight through a sample in which all particles have been removed fromsuspension will be maximum, and the rate of change of intensity=0. WhenI is maximum (or some other predetermined threshold), or dl/dt=0 (orsome other predetermined threshold), or both, the separation process canbe terminated.

Monitor the quality of the suspension based on dl/dt: In a givenmagnetic field gradient, a sample containing larger particles, oragglomerates of smaller particles will be removed from the suspensionfaster than smaller, non-agglomerating particles.

Monitor the relative concentration of a solution: If, over the progressof a separation, I remains relatively low and dl/dt=0, the needleapparatus might be saturated with magnetic particles. Additionalseparations might be required until I reaches a relative maximum.Alternatively, the size of the magnetic needle apparatus can beconfigured to increase the separation efficiency.

Optimize the configuration of the magnetic needle apparatus based ondl/dt: a sample containing larger particles will be removed from thesuspension at lower magnetic gradients relative to a sample containingsmaller particles. In a given configuration, this can result in asignificantly different separation time or efficiency for large vs.small particles. For smaller particles, the magnetic needle apparatuscan be configured to achieve higher magnetic gradients for optimal, moreefficient separations as determined by monitoring dl/dt.

Although the present invention has been described in terms of variousexemplary embodiments for purposes of illustration, those of ordinaryskill in the art will appreciate that various modifications andimprovement may be made to the described embodiments without departingfrom the scope of the invention.

What is claimed is:
 1. A method for the separation of magnetic particlesfrom a suspension, comprising: (a) placing a magnetic needle into thesuspension; (b) subjecting the suspension to incident light at a firstwavelength; (c) determining a measure of light at the first wavelengthafter interaction with the suspension; and (d) removing the magneticneedle from the suspension responsive to the measure of light.
 2. Themethod of claim 1, wherein determining a measure of light comprisesdetermining a measure of light intensity after light has traveledthrough the suspension.
 3. The method of claim 2, wherein the measure oflight intensity is a measure of the rate of change of light intensity.4. The method of claim 2, wherein the measure of light intensity is ameasure of the intensity of light collected compared to the intensity ofincident light.
 5. The method of claim 3, wherein step (d) comprisesremoving the magnetic needle when the rate of change is below apredetermined threshold.
 6. The method of claim 5, wherein thepredetermined threshold is zero.
 7. The method of claim 4, wherein step(d) comprises removing the magnetic needle when the ratio of theintensity of light collected to the intensity of incident light is belowa predetermined threshold.
 8. The method of claim 2, wherein step (d)comprises removing the magnetic needle when the intensity of lightcollected is below a predetermined threshold.
 9. The method of claim 1,wherein step (a) comprises placing a removable sheath over the magneticneedle prior to placing the magnetic needle into the suspension, andfurther comprising (e) removing the sheath from the magnetic needleafter removing the magnetic needle from the suspension.
 10. The methodof claim 1, wherein the first wavelength is in the visible range. 11.The method of claim 1, wherein step (c) comprises determining a measureof light at the first wavelength after a portion of the incident lighthas been absorbed, reflected, scattered, or a combination thereof, alongan optical path through the suspension.